The present invention relates to floating structures and more particularly concerns methods and apparatus for improving stabilization of such structures.
In various types of operations, such as scientific surveys, oil and gas well drilling and production, floating platforms are required because of water depths and other conditions, both physical and economic. For such floating platforms, maximum stability is required. The platform should move as little as possible in response to the forces exerted by waves. The use of a conventional ship type hull is hardly satisfactory because of excessive motion of this type of structure in response to sea motion. A type of platform commonly called a semi-submersible platform has been developed to provide for maximum stability in the presence of rough water and weather conditions. A semi-submersible platform typically comprises a number of submerged hulls and an elevated superstructure. A number of vertical columns support the superstructure from the submerged hulls. With the platform at rest in still water, the weight of the platform is exactly balanced by the hydrostatic buoyancy force acting on the submerged part of the platform. Accordingly, the platform will normally float with the hull and lower portions of the vertical columns submerged and the deck floating clear of the water.
Nevertheless, even the semi-submersible platform moves with water motion. Heave in particular is a problem in rough water areas. The platform, of course, is subject to other motions, such as roll and pitch, roll being an angular displacement about a longitudinal axis, pitch being an angular displacement about a transverse axis and heave being vertical linear displacement of the platform.
Resonant motion occurs when the natural periods of the various motion components, heave, roll and pitch, are substantially equal to the periods of the disturbing forces which cause such heave, roll and pitch. For avoidance of resonant motion a semi-submersible platform is superior to a surface vessel, particularly because its natural resonant period may be more readily controlled in design of the platform. In analysis of the heaving characteristics of the semi-submersible, one may consider the vertical force exerted by the water as the sum of forces acting upwards on the bottom of the vertical columns and those acting upwards on the submerged hull structure, which is often in the form of horizontally extending pontoons interconnecting the lower ends of the vertical columns.
Operational draft of such a platform is substantial, in the order of 50 to 70 feet, which causes oscillatory heaving forces that act on column bottoms and pontoons to be relatively small since pressure fluctuations decrease with depth. Further, it is possible to choose the dimensions of the vertical columns and the pontoons or submerged hull so that at some predetermined wave period the resultant vertical force upon the platform approaches zero, leaving only small residual drag forces. Thus, it is common in design of semi-submersible platforms to employ a relative large displacement in the submerged hull and to employ relatively small displacement in the vertical columns. This will increase the resonant period of platform heave so that it is longer than the longest period of naturally occurring waves that may be reasonably expected in a particular area. If the resonant period of the platform is caused to be about eighteen seconds, heave resonance effects are eliminated for seas of any size having a wave period smaller than about sixteen seconds.
Major efforts in platform design are directed toward avoidance of resonance because of the severe adverse effects of resonance. Motions of the platform in response to seas having a period equal to the resonant period of the platform are greatly increased. Thus heave displacement at resonance may be several times greater than the maximum heave displacement, for comparable wave height, at other, non-resonant wave periods.
However, design considerations leading to an increased resonant period result in platforms that are more costly, larger and in some respects less stable. This problem is recognized by the patentees Schuler et al in U.S. Pat. No. 3,391,666, who suggest a solution which entails varying stability of the platform by a variation in the water plane area (and thus the inertia of the platform column). This is said to be achieved in the Schuler patent by employing hollow columns that may be flooded with sea water or charged with air thus effectively varying the water plane area of the individual columns. This is an expensive and complex solution and yet may not effectively control platform motion.
Many other types of structures have been employed for controlling stability and minimizing motion of floating platforms including hydrodynamic structures such as shown in U.S. Pat. Nos. 3,318,275, 3,349,740, 2,190,617, passive or active hydrostatic structures such as shown in U.S. Pat. Nos. 3,159,130, 3,083,671, 3,160,135, 3,537,412, 3,207,110, 2,889,795 and also arrangements that are anchored or tethered to the bottom, such as shown in U.S. Pat. Nos. 2,972,973, 3,654,886, 3,702,105 and 3,566,608. However, these systems all fail to effectively handle the problem of excessive amplitude of platform displacement in the presence of waves of a period substantially equal to the natural resonant period of the platform. Thus it is still necessary to design the platform so that its natural resonant period is above the longest wave period to be expected. Further, many of these systems require arrangements of tanks and interconnecting conduits which themselves are resonant at certain frequencies and these will operate optimally solely at a preselected wave period.
Because of the size of the semi-submersible platforms, the disturbing forces exerted by the water are large and thus forces needed to counteract such disturbing forces may be excessive. Dynamic systems employing principles of prior art arrangements may economically exert forces of a magnitude such as to have too little effect at resonance. In other words, it is more feasible according to prior arrangements to avoid resonance rather than to counteract resonant amplitudes.
Accordingly, it is an object of the present invention to provide motion stabilization of a floating structure which avoids or minimizes problems described above.